An explicit analytic expression for the scalar of curvature power
spectrum can be obtained when neglecting the coupling between inflaton
and radiation perturbations (i.e., taking \(c=0\)  in
Eq. (REF )) and it was determined in
Ref. [1]}. The result in this case is
well-approximated by [1]}
\(\Delta _{{\cal R}} \simeq \left(\frac{ H_{*}^2}{2\pi \dot{\phi }_*}\right)^2 \left(1+2{n_{BE}} + \frac{2\sqrt{3}\pi Q_*}{\sqrt{3+4\pi Q_*}}{T_*\over H_*}\right) ,\) 
